Device and method for internal combustion engine control

ABSTRACT

A system for an engine is shown that includes an upstream emission control device and a downstream emission control device. The upstream device has a washcoat having a predetermined amount of precious metal disassociated with NOx storage material. In one example, the upstream emission control device located in a close-coupled location receiving untreated exhaust gasses from the engine. A controller is utilized for adjusting parameters of the engine. For example, the controller operates the engine lean, with periodic rich operation to purge stored NOx during at least a portion of low engine airflow and low exhaust temperature conditions. The controller also operates the engine near stoichiometry during at least a portion of other conditions.

BACKGROUND

Lean burn operating engines utilize emission control devices coupled tothe engine to store NOx while operating lean, and then to reduce storedNOx when the engine operates rich.

Various catalyst formulations have been proposed for increasing the NOxstorage capacity of emission control devices.

However, the inventors here have recognized a disadvantage with suchcatalysts. In particular, these catalysts typically have a lower maximumtemperature durability, and also provide reduce HC and CO conversionefficiency at higher temperatures. As such, these catalysts aretypically located downstream of another catalyst, thereby exposing themto lower temperatures. However, by placing them downstream, this createsvarious problems related to inefficient purging (since all of theoxidants in the upstream catalyst must first be purged before thedownstream catalyst can be purged).

SUMMARY OF THE INVENTION

The above disadvantages are overcome by a system for an engine,comprising:

an upstream catalyst that can store oxidants when the engine isoperating lean and release and reduce said stored oxidants whenoperating the engine stoichiometric or rich;

a downstream catalyst that can store oxidants when the engine isoperating lean and release and reduce said stored oxidants whenoperating the engine stoichiometric or rich; and

a controller for determining whether exhaust temperature is less than afirst value; determining whether engine airflow is less than a secondvalue; and enabling lean operation based at least on said first andsecond determinations.

In one example, the upstream catalyst has a washcoat having apredetermined amount of precious metal disassociated with NOx storagematerial. Accordingly, such an upstream catalyst can provide higherconversion efficiency at higher exhaust temperatures and therefore isable to endure being in an upstream location in the exhaust system. Onthe other hand, above certain temperatures and engine airflows, the NOstorage capacity and efficiency fall of such catalysts can fall belowacceptable values in some cases. Nevertheless, since such catalysts canachieve efficient operation when the engine is operated at nearstoichiometry, the present invention simply disables lean operation inthese regions and operates the engine at near stoichiometry. As such, itis possible to place these catalysts in an upstream location and achievevarious benefits, such as efficient NOx purging and improved coldstarting emission.

Note that the exhaust temperature utilized in the present invention canbe implemented in various ways. For example, the controller can useeither measured or estimated exhaust temperatures. Further, thecontroller can use exhaust gas temperatures, exhaust manifoldtemperatures, catalyst temperatures, or any other such values todetermine whether to enable lean operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 show an engine system configuration according to an exampleembodiment of the invention;

FIG. 2 shows alternative exhaust configuration according to anotherexample embodiment of the invention;

FIG. 3 shows various graphs illustrating aspects of an exampleembodiment of the invention;

FIGS. 4-5 are high level flowcharts illustrating operation according anexample embodiment of the invention;

FIG. 6 shows a graph illustrating example performance according to anaspect of the invention;

FIG. 7 shows an example exhaust configuration;

FIG. 8 is a schematic of engine speed versus brake mean effectivepressure (BMEP) at different air/fuel ratios;

FIG. 9 is a graph showing NOx conversions and NOx release of catalystswith different oxygen storage capacity (“OSC”) during lean to richtransitions;

FIG. 10 is a schematic view of a catalyst system that incorporates thepresent invention, showing a first catalyst having two zones and asecond catalyst;

FIG. 11 a is a graph of conversion efficiency for converting HC, CO andNOx for the zoned catalyst system described in example 1; and FIG. 11 bis a graph of NOx adsorption versus temperature for the zoned catalystsystem described in example 1;

FIG. 12 a is a graph of conversion efficiency for converting HC, CO andNOx for the comparative catalyst described in example 2, FIG. 12 b is agraph of NOx adsorption versus temperature for the comparative catalystdescribed in example 2; and

FIG. 13 is a schematic view of a catalyst system that incorporates thepresent invention, showing a catalyst having two layers.

DETAILED DESCRIPTION

Direct injection spark ignited internal combustion engine 10, comprisinga plurality of combustion chambers, is controlled by electronic enginecontroller 12. Combustion chamber 30 of engine 10 is shown in FIG. 1including combustion chamber walls 32 with piston 36 positioned thereinand connected to crankshaft 40. In this particular example piston 36includes a recess or bowl (not shown) to help in forming stratifiedcharges of air and fuel. Combustion chamber 30 is shown communicatingwith intake manifold 44 and exhaust manifold 48 via respective intakevalves 52 a and 52 b (not shown), and exhaust valves 54 a and 54 b (notshown). Fuel injector 66 is shown directly coupled to combustion chamber30 for delivering liquid fuel directly therein in proportion to thepulse width of signal fpw received from controller 12 via conventionalelectronic driver 68. Fuel is delivered to fuel injector 66 by aconventional high pressure fuel system (not shown) including a fueltank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 viathrottle plate 62. In this particular example, throttle plate 62 iscoupled to electric motor 94 so that the position of throttle plate 62is controlled by controller 12 via electric motor 94. This configurationis commonly referred to as electronic throttle control (ETC) which isalso utilized during idle speed control. In an alternative embodiment(not shown), a bypass air passageway is arranged in parallel withthrottle late 62 to control inducted airflow during idle speed controlvia a throttle control valve positioned within the air passageway.

Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48upstream of catalytic converter 70. In this particular example, sensor76 provides signal EGO to controller 12 which converts signal EGO intotwo-state signal EGOS. A high voltage state of signal EGOS indicatesexhaust gases are rich of stoichiometry and a low voltage state ofsignal EGOS indicates exhaust gases are lean of stoichiometry. SignalEGOS is used to advantage during feedback air/fuel control in aconventional manner to maintain average air/fuel at stoichiometry duringthe stoichiometric homogeneous mode of operation.

Conventional distributorless ignition system 88 provides ignition sparkto combustion chamber 30 via spark plug 92 in response to spark advancesignal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either ahomogeneous air/fuel mode or a stratified air/fuel mode by controllinginjection timing. In the stratified mode, controller 12 activates fuelinjector 66 during the engine compression stroke so that fuel is sprayeddirectly into the bowl of piston 36. Stratified air/fuel layers arethereby formed. The strata closest to the spark plug contains astoichiometric mixture or a mixture slightly rich of stoichiometry, andsubsequent strata contain progressively leaner mixtures. During thehomogeneous mode, controller 12 activates fuel injector 66 during theintake stroke so that a substantially homogeneous air/fuel mixture isformed when ignition power is supplied to spark plug 92 by ignitionsystem 88. Controller 12 controls the amount of fuel delivered by fuelinjector 66 so that the homogeneous air/fuel mixture in chamber 30 canbe selected to be at stoichiometry, a value rich of stoichiometry, or avalue lean of stoichiometry. The stratified air/fuel mixture will alwaysbe at a value lean of stoichiometry, the exact air/fuel being a functionof the amount of fuel delivered to combustion chamber 30. Controller 12adjusts fuel injected via injector 66 based on feedback from exhaust gasoxygen sensors (such as sensor 76) to maintain the engine air-fuel ratioat a desired air-fuel ratio.

Second emission control device, which is described in more detail below,is shown positioned downstream of the first emission control device 70.Devices 70 and 72 each contain catalyst of one or more bricks. However,in an alternative embodiment, devices 70 and 72 can be different bricksin the same canister, or separately packaged. Several exampleembodiments of Devices 70 and 72 are described below.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, anelectronic storage medium for executable programs and calibration valuesshown as read only memory chip 106 in this particular example, randomaccess memory 108, keep alive memory 110, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:measurement of inducted mass air flow (MAF) from mass air flow sensor100 coupled to throttle body 58; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a profile ignitionpickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft40; and throttle position TP from throttle position sensor 120; andabsolute Manifold Pressure Signal MAP from sensor 122. Engine speedsignal RPM is generated by controller 12 from signal PIP in aconventional manner and manifold pressure signal MAP provides anindication of engine load.

In this particular example, temperature T1 of device 70 and temperatureT2 of device 72 are inferred from engine operation. In an alternateembodiment, temperature T1 is provided by temperature sensor 124 andtemperature T2 is provided by temperature sensor 126.

In another alternative embodiment, a port fuel injected engine can beused where injector 66 is positioned in intake manifold 44 to injectedfuel toward valve 52 a and chamber 30.

Referring now to FIG. 2, a simplified block diagram describing analternative exhaust system according to one aspect of the presentinvention is illustrated. In FIG. 2, engine 10 is shown coupled toupstream catalyst 70 and downstream catalyst 72. As described below,catalyst 70 contains elements x, y and z, and catalyst 72 containselements a, b and c. In this way, the engine is operated lean and NOxand oxygen (oxidants) are stored primarily in catalyst 70. Then, oncethe NOx and oxygen capacity of catalyst 70 is neared (or the amount ofNOx breakthrough reaches a predetermined level) the engine is switchedto rich or stoichiometric operation to reduce the oxidant stored incatalyst 70 and any breakthrough oxidants stored in catalyst 72.However, according to the present invention, the engine is operated richprimarily to reduce the oxidants stored in catalyst 70 and once this issubstantially accomplished the engine is returned to lean operation.Note that this is just one example of operation according to the presentinvention and various other modes can be used, particularly, the leanoperation can be extended to allow some oxidant storage in catalyst 72.However, if catalyst 72 is a catalyst optimized for oxygen storagerather than NOx storage, the engine is typically operated lean so longas the NOx is being stored in catalyst 70. In this way, efficient NOxpurges can be performed since little fuel is wasted purging storedoxygen in catalyst 72 since it is downstream of catalyst 70.

FIG. 2 also shows three sensors (S1, S2, and S3) in the system. S1 iscoupled between the engine 10 and catalyst 70. Sensor S2 is coupledbetween catalyst 70 and catalyst 72. Sensor S3 is coupled downstream ofcatalyst 72.

The sensors are generically described as sensors S1 through S3 and canbe various types of sensors such as, for example: heated exhaust gasoxygen sensors, universal exhaust gas oxygen sensors, NOx sensors,combined NOx/UEGO sensors, or temperature sensors, or any combination ofthe above sensors.

Referring now to FIG. 3, various graphs are shown illustrating operationaccording to the present invention. The first Figure shows the requestedair fuel ratio is a function of the requested torque and engine speed.The requested air fuel regions are broken into a lean region, astoichiometry region, and a rich region. Note that stoichiometric asused herein refers to oscillatory feedback air-fuel ratio control aboutthe stoichiometric air fuel ratio.

The second graph of FIG. 3 shows whether lean operating conditions areenabled based on exhaust or engine space velocity. In other words, evenif the requested torque and engine speed are in the lean region of thefirst figure, the stoichiometric mode will be selected if the spacevelocity is greater than the threshold level L1 on FIG. 3.

The third graph of FIG. 3 shows whether lean operation is enabled basedon exhaust gas temperature (estimated or measured from a sensor atlocation of sensor S1). In other words as with respect to the secondgraph of FIG. 3, even if the requested torque and speed are in the leanregion of the first graph, if the exhaust gas temperature is greaterthan the threshold L2, the lean operation is disabled and the engine isoperated at the near stoichiometric value. Note that both L1 and L2 canbe adjusted based on engine operating conditions.

Referring now to FIG. 4, a routine describing operation according to thepresent invention is illustrated. In general terms, the routinedescribes how the engine is operated to optimize engine fuel economy byperforming efficient NOx purging using an upstream catalyst capable ofsignificant NOx storage and a downstream catalyst optimized foroperation near stoichiometry. However, as described above, there arevarious alternate embodiments of the present invention and the routinedescribed in FIG. 4 is applicable generally to these.

First, in step 410, the routine determines whether lean operation isenabled. In other words, the routine determines the desired or requestedair-fuel ratio based on the requested torque and engine speed. Then, theroutine determines whether the space velocity is greater than thethreshold L1 or whether the exhaust gas temperatures is greater thanthreshold L2. If neither the space velocity gas temperature are abovethe respective thresholds, and the requested torque and engine speedindicate a requested lean air-fuel ratio, then the answer to step 410 isyes, and the routine continues to operate the engine lean in step 412.

Next, in step 414, the routine determines whether a NOx purge isrequested. In particular, the routine estimates the amount of NOx storedin catalyst 70 and determines whether this estimate is greater than thethreshold B1. The amount of NOx stored in catalyst 70 is estimated basedon various operating conditions such as, for example: engine speed,engine load, temperature, and space velocity. Alternatively, the routinecan determine if a NOx purge is requested by calculating whether thetailpipe NOx emissions are above a predetermined threshold, or whetherthe amount of NOx exiting catalyst 70 is above a threshold value. Instill another alternative embodiment, the routine can determine whethera NOx purge should be requested based on the storage efficiency of NOxin catalyst 70.

When the answer to step 414 is no, the routine returns to step 412 andcontinues the lean operation, provided the lean operating conditionsdetermined in step 410 are still present. Alternatively, when the answerto step 414 is yes, the routine continues to step 416 where the engineis transitioned to stoichiometric or rich operation to perform the NOxpurge.

During the NOx purge, the routine determines in step 418 whether to endthe NOx purge. In one example, the routine determines whether theintegral of an exhaust gas oxygen sensor (as sensor S2) is greater thana preselected value. In other words, the routine determines whether anintegrated amount of rich exhaust gases have exited catalyst 70.Alternatively, the routine can determine whether a predetermined richtime interval, or a predetermined number of rich engine cycles has beencompleted. Alternatively, the routine can determine whether an output ofan exhaust gas oxygen sensor as sensor S2 indicates a value richer thana preselected value.

When the answer to step 418 is no, the routine continues to step 416 andcontinues to operate in the NOx purging condition. Alternatively, whenthe answer to step 418 is yes, the routine continues to step 420 to endthe NOx purge.

Continuing with FIG. 4, in step 422, the routine determines whether leanoperation is still enabled by determining factors such as describedabove herein with regard to step 410. When the answer to step 422 isyes, the routine transitions to return the engine to lean operation instep 426. Alternatively, when lean operation is no longer requested orenabled, the routine continues to step 424 to transition tostoichiometric control.

The stoichiometric mode of operation, as described above, refers tooperating the engine with the desired air-fuel ratio oscillating aroundthe stoichiometric value (within very close limits to stoichiometry, forexample, plus or minus 5%).

Referring now to FIG. 5, the routine is described for controlling enginecold starts. In general terms, the routine controls the engine, aftercranking, to operate with a slightly lean air-fuel ratio with retardedignition timing to heat the ignition control device 70 and 72. Thisoperation is continued until the upstream catalyst temperature reaches athreshold temperature T1. After this time, the engine is operatedstoichiometric or rich to purge NOx stored during the initial leanoperation.

First, in step 510, the routine determines whether cold start conditionshave been detected. This can be based on various conditions such as, forexample, time since engine start, time since the last engine start,exhaust temperature, and other conditions. When the answer to step 510is yes, the routine continues to step 512. In step 512, the routinedetermines whether the engine has been started. When the answer to step512 is no, the routine continues monitoring whether the engine has beenstarted. Various parameters can be used to detect an engine start, suchas operating the engine with a predetermined number of firings,determining whether the engine speed has reached a threshold value,determining whether the engine has been turning greater than apredetermined time, and various others.

When the answer to step 512 is yes, the routine continues to step 514.In step 514, the routine operates the engine slightly lean with retardedignition timing to generate heat for heating the emission controldevices and exhaust gas system. Next, in step 516, the routinedetermines whether the upstream temperature is greater than thethreshold T1. Various methods can be used in step 516, such asdetermining whether estimated exhaust gas temperature has reached apredetermined threshold, whether catalyst temperature of upstreamcatalyst 70 has reached a predetermined temperature, whether an estimateof catalyst temperature has reached a predetermined threshold, andvarious others. When the answer to step 516 is no, the routine returnsto step 514 to continue lean operation with retarded ignition timing. Inthis way, the exhaust catalysts are heated to increase theirtemperature. Alternatively, when the answer to step 516 is yes, theroutine operates the engine stoichiometric or rich to purge NOx storedin the emission control devices.

From step 518, the routine continues to step 520 where a determinationis made as to whether to end the purge of stored NOX. This can bedetermined in various ways such as, for example, operating rich for apredetermined time, using a sensor located downstream of the emissioncontrol device to be purged of NOx, estimating the amount of fueldelivered to the exhaust system, and various others. When the answer tostep 520 is yes, the routine transitions in step 522 to nearstoichiometric or lean operation based on operating conditions.Otherwise, when the answer to step 520 is no, the routine returns tostep 518 to continue the NOx purge.

In this way, it is possible to lower vehicle tailpipe NOx emissionsduring cold start since the NOx can be stored in the upstream catalystand then purged by rich operation once the predetermined temperature hasbeen reached.

Referring now to FIG. 6, graphs illustrating operation according to thepresent invention are described. The top graph of FIG. 6 illustrates thegradual increase of catalyst temperatures for the upstream anddownstream catalysts 70 and 72. In particular, the solid line shows thetemperature of upstream catalyst 70, whereas the dashed line shows thetemperature of downstream catalyst 72. As expected from a cold start,the upstream catalyst heats up quicker than the downstream catalyst. Thepresent invention uses this to advantage by retaining oxidants such asNOx in the upstream catalyst and then purging the stored NOx before thestorage capacity of the upstream catalyst has been depleted, therebyminimizing fuel used to reduce oxidants, such as oxygen, stored in thedownstream catalyst 72. Furthermore, during a cold start, the presentinvention uses the location of the upstream catalyst to advantage byretaining NOx produced at engine starting conditions before the time t1.After the time t1 (when the upstream catalyst has reached temperatureT1) the engine is transitioned to rich to purge the NOx stored in theupstream catalyst as described in the lower graph of FIG. 6. Inparticular, the lower graph of FIG. 6 shows the engine air-fuel ratiocorresponding to the temperatures in the top graph. As shown, once theengine exits engine cranking and run-up, the engine is operated slightlylean (between about 14.6 and 15:1 air-fuel ratios). During this time NOxemitted from the engine is stored in catalyst 70. After time t1, theengine is transitioned to rich as described above to purge the storedNOx.

Note that the catalysts ability to store and reduce NOx is a function oftemperature, and if the NOx storage was primarily used in the downstreamcatalyst 72, the engine would have to wait until time t2 to be able totake advantage of the NOx storage ability. However, according to thepresent invention, by using the catalyst configuration of the presentinvention it is possible to earlier take advantage of this NOx storageand purging ability, and thereby reduce NOx emissions during an enginecold start.

FIG. 7 shows the catalysts 70 and 72 in a close-coupled location to theengine. Further, the figure shows a dual-pipe configuration, where thereare two sets of catalysts 70A,B and 72A,B. The two exhaust paths A and Bdo not mix in this example. However, in an alternative embodiment, aY-pipe configuration can be used where the gasses come together.

In an alternative example embodiment, the engine is run understoichiometric conditions most of the time, except under low load (brakemean effective pressure (BMEP)<1.2 bar), low engine speed (RPM<1750)conditions, when the engine is run under stratified-charged orhomogenous lean conditions. Lean operation can also be selected based onengine torque and engine speed. The operation diagram of a stratifiedcharged engine is schematically shown as FIG. 8. Also shown in FIG. 8 isthe operation range when the engine is run under stoichiometriccondition and under full load with air to fuel ratio under net fuel richcondition.

For engines operated as above, and any other internal combustion enginedesigned to operate at least partially under lean conditions, thepresent invention provides a catalyst system capable of reducing engineemissions.

As set forth above, FIG. 9 depicts typical lean NOx release during thelean-rich transition of an emission control device that is capable ofstoring oxidants, such as oxygen and NOx. The NOx released during thepurge cycle of the catalyst is believed to occur due to the exothermicheat generated from the oxidation of reductants CO, HC and H₂ by theoxygen released from the oxygen storage material.

FIG. 10 depicts an example embodiment of the catalyst system 19, asystem for the treatment of emissions both under stoichiometricoperation and under lean conditions (e.g., during idling and low speedcruises). As shown, the catalyst system 19 includes two catalysts 70, 72in a close-coupled location. The forward catalyst 70 is optimized tofunction when the engine 10 is operated under lean conditions. Theforward catalyst 70 will store excess NOx during lean operation and thenrelease and convert the NOx when the engine 10 switches to richconditions. The downstream catalyst 72 is optimized to effectivelyconvert HC, CO, and NOx under stoichiometric operations, as well asconvert any remaining lean NOx emissions not adsorbed by the forwardcatalyst 70.

The forward catalyst 70 consists of two zones 18, 20. It should be notedthat this invention also contemplates the use of layers in place ofzones. In one embodiment of the invention, these different layer orzones make it possible to physically segregate oxidation components toprovide NOx storage, while at the same time providing efficient HC/COoxidation activity. Note that different phases could also be used.

In one approach, two components are included in the catalyst washcoatsuch that it would have both NOx storage and high HC/CO conversionactivity in the same catalyst. To achieve efficient NOx storage, ahighly interacted NO oxidation catalyst and NO₂ storage material areused. This is typically a precious metal (Pt) and Ba, but other suitablematerials could be substituted, such as cesium or potassium. This allowsefficient transfer of the oxidized NO₂ to the storage material.Unfortunately, this reduces the activity of the Pt to oxidize HC and CO.Hence, for good oxidation activity, Pt and/or Pd are placed on Al₂O₃ orCe/Zr where the Pt/Pd will have good oxidation activity. To create acatalyst with both HC/CO activity and NOx storage, the two phases aresegregated so that the Ba does not interfere (or does so less than apreselected amount) with the oxidation activity of the Al₂O₃ phase. Thiscan be accomplished by creating separate phases of the material wherethe Pt/Pd is first fixed on one support (Al₂O₃ or Ce/Zr mixed oxide) andPt/Ba mixture is fixed on an Al₂O₃ support. These separate phases couldthen be either mixed together and washcoated or preferably could bewashcoated as two distinct layers. Another feature of this process wouldbe to use a solvent in which none of the active materials hasappreciable solubility so that they would not be mixed when a slurry wasprepared in the washcoat process. In this way, the problems with NOxrelease are overcome. In other words, from a macro viewpoint, a selectedamount of precious metal is placed in the washcoat that is notassociated with NOx storage materials such as Ba. In one example,between 30-70% (by mass) of the precious metal is placed in the washcoatthat is not associated with NOx storage materials. Specific rangesinclude: 10-20, 20-30, 30-40, 40-50, 60-70, and/or 80-90. Anotherexample includes 50-80%.

Zone 18 is, in one example, made from a foraminous substrate, such as analumina substrate that functions as a carrier for the catalyst materialand acts as a passage for the exhaust gas stream. The substrate iscoated with a high loading of a precious metal (PM) such as Pt, Pdand/or Rh. Zone 18 further includes oxides of aluminum, alkali metals oralkaline earth metals, or a combination of the two, such as bariumoxide, magnesium oxide, or potassium oxide. The total amount of alkalimetals or alkaline earth metals or a combination of the two ranges from2-15 (wt) %. Zone 18 is, however, devoid of cerium. In one embodiment,zone 18 of the forward catalyst 70 consists of platinum and rhodiumloaded on an alumina substrate in the ratio of 5:1 to 25:1 with a totalloading of approximately 60-300 g/ft³. In embodiment, the Rh is anchoredon 3-5% (wt) ZrO₂ particles. The Al₂O₃ substrate is stabilized byapproximately 1-8% (wt) La₂O₃.

Zone 20 of the forward catalyst 70 is also made from a foraminoussubstrate, usually alumina that functions as a carrier for the catalystmaterial. Zone 20 includes oxides of aluminum, alkali metals, alkalineearth metals, rare earth metals, or combinations thereof and a lowerloading of precious metals (PM) than zone 18. Zone 20 may optimallyinclude oxides of zirconium. In one embodiment, the alumina substrate inzone 20 is coated with a solution containing 10-40% (wt) of BaO and/orMgO on an Al₂O₃ washcoat. In one embodiment, zone 20 of the forwardcatalyst 70 consists of Pt and Rh loaded on an alumina substrate in theratio of 1:1 10:1, with a total loading of about 10-100 g/ft³. Zone 20can include a small amount of mixed oxides of zirconium and cerium.

In alternative embodiment, forward zone 18 of catalyst 70 includesbetween 0.1 and 6% (wt) BaO and/or MgO washcoat on alumina. In aspecific example, between 1 to 3% (wt) of BaO is utilized. In anotherexample, 3 to 9% is utilized. This allows a predetermined amount of NOxstorage, but also allows high retention of HC and CO conversion duringstoichiometric operation. Thus, by keeping amount of barium in theseexample ranges, this allows efficient stoichiometric operation (high HCand CO conversion) at higher temperatures, and therefore catalyst 70 canbe located upstream. Note that the upstream catalyst 70 can be closecoupled to the engine exhaust and receive untreated exhaust gasses fromthe engine. I.e., exhaust gasses that have not passes through any otherupstream catalytic converters in a separate canister.

The downstream catalyst 72 is manufactured in a method similar to zone18. The downstream catalyst 72 also contains a catalyst material, PM-Rh,wherein the precious metals (PM) can be platinum, palladium orcombinations thereof, preferably platinum and rhodium in a ratio ofPt/Rh 5:1-15:1. In addition to precious metals (PM), this downstreamcatalyst 72 includes oxides of aluminum, alkali metals or alkaline earthmetals and a high concentration of mixed oxides of zirconium and cerium.In one embodiment, the total loading of the catalyst material in thedownstream catalyst 72 is about 10 g/ft³-120 g/ft³. In one embodiment,both Pt and Rh are anchored on 5-30% (wt) high surface area Ce/Zr withhigh O₂ kinetics (e.g., Ce/Zr=50:50 molar ratio). The alumina washcoatis optionally also stabilized by 2-15 (wt) % BaO. Optionally, thedownstream catalyst 72 may also include hydrogen sulfide emissionsuppressants such as nickel oxide.

The foregoing catalyst system 19 reduces the oxygen storage function inthe front zone of the first catalyst, so that NOx release is minimized.The reduction of oxygen storage function in the front zone is alsobelieved to minimize the fuel required to purge the catalyst asdescribed herein. The forward catalyst 70 functions to store the NOxproduced by engine 10 during lean operation. When the engine 10 isswitched from lean to rich conditions for the purge of the firstcatalyst, there is minimum stored oxygen so that the oxidation reactionsuch as CO with O₂ does not cause an exothermic reaction in zone 18 andthus there is minimal release of unreduced NOx. The first catalyst 70can be purged and the stored NOx converted when an engine control moduledetermines that the NOx storage capacity of the first catalyst isapproached.

Optionally, an exhaust gas oxygen sensor 22 is positioned upstream ofthe catalyst 72 between the forward catalyst 70 and the downstreamcatalyst 72, as shown in FIG. 3. Under this arrangement, there is aminimal fuel economy penalty from the oxygen storage capacity (OSC) ofcatalyst 72. Using this arrangement, the oxygen storage function of thedownstream catalyst 72 is minimized so that minimal NOx release occursdue to the exothermic reaction described above. Additionally, thecerium-free first zone and the lower loading of cerium in the secondzone of the forward catalyst help to minimize the fuel penaltyattributed to the purges.

FIGS. 11 a and 11 b show the experimental data on the effectiveness ofthe catalyst system 19. FIG. 11 a shows the effectiveness in treatingsteady state HC, CO, and NOx and steady state lean NOx after being agedon a pulsator at 850° C. for 50 hr with a space velocity of 50,000 hr⁻¹.As seen in FIG. 11 a, at λ≧1, HC, CO and NOx conversion is between95-100% efficient. The data in FIG. 11 a was obtained using thefollowing feed gas: 500 ppm NO, 1500 ppm HC, 1% CO, 10% CO₂, 10% H₂O anda varying amount of O₂. FIG. 11 b shows the steady state lean NOxperformance of an 850° C. aged catalyst system with a volumetric spacevelocity 30,000 hr⁻¹. As seen in FIG. 11 b, because of this invention,NO is adsorbed over a wide range of operating temperatures, from 200°C.-600° C. and thus more effective for NOx reduction under a wide rangeof temperature parameters. The data in FIG. 11 b was obtained using thefollowing feed gas: 500 ppm NO, 1500 ppm HC, 1% CO, 10% CO₂, 10% H₂O and6% O₂.

This catalyst system is expected to be used in automotive vehicles foremission treatment in the exhaust gas system where it functions tooxidize hydrocarbons, carbon monoxide, and reduce nitrogen oxides todesired emission levels.

FIG. 12 a is a graph illustrating CO, NOx and HC efficiency for thecatalyst described in Example 2. FIG. 12 a further illustrates thelambda sweep of the catalyst described in Example 2.

FIG. 12 b is a graph illustrating the temperature profile of the leancatalyst prepared for Example 2, aged as provided in FIG. 11 b.

Each catalyst is deposited or washcoated on a substrate (mechanicalcarrier) made of a high temperature stable, electrically insulatingmaterial such as cordierite, mullite, etc. A mechanical carrier is, inone example, comprised of a monolithic magnesium aluminum silicatestructure, i.e., cordierite, although the configuration is not criticalto the catalyst system of this invention.

The surface area of the monolithic structure provides 50-1000 meterssquare per liter structure, as measured by nitrogen adsorption. Celldensity should be maximized consistent with pressure drop limitationsand is, in one example, in the range of 200-800 cells per square inch ofcross-sectional area of the structure. The substrate may be in anysuitable configuration, often being employed as a monolithic honeycombstructure. Other configurations useful in this invention and suitable inan exhaust gas system will be apparent to those skilled in the art inview of the present disclosure.

Various techniques for providing an oxide washcoat on a substrate can beused. Generally, a slurry of the mixed metal oxide particles andoptionally stabilizer particles are coated on a substrate, e.g., addedby dipping or spraying, after which the excess is generally blown off.After the slurry of mixed metal oxide particles are coated on thesubstrate, the substrate is heated to dry and calcine the coating,generally at a temperature of about 600° C. for about 2-3 hours.Calcining serves to develop the integrity of the ceramic structure ofthe washcoated oxide coating. The total amount of the oxide washcoatcarried on the substrate is about 10-30% (wt), based on the weight ofthe substrate coated. Several coatings of the substrate and the washcoatmay be necessary to develop the desired coating thickness/weight on thesubstrate.

The precious metals may be provided on the calcined oxide coating by anytechnique including the well known wet impregnation technique fromsoluble precious metal precursor compounds. Water soluble compounds canbe used, including, but not limited to, nitrate salts and materials forplatinum like chloroplatinic acid. As known in the art, afterimpregnating the washcoat with the precursor solution, it is dried andheated to decompose the precursor to its precious metal or preciousmetal oxide. The precursor may initially decompose to the metal but beoxidized to its oxide in the presence of oxygen. While some examples ofprecious metal precursors have been mentioned above, they are not meantto be limiting. Still other precursor compounds would be apparent tothose skilled in the art in view of the present disclosure.

In addition to this incorporation from a liquid phase, the preciousmetal, such as platinum, may be provided by sublimation of platinumchloride or other volatile platinum salts, by a solid state exchange inthe 300-500° C. temperature range using labile platinum compounds. Thereis no criticality to the type of precursor compounds that may be used toprovide the precious metal according to this invention.

Additionally, co-precipitation techniques can also be used to form thecatalyst. According to such techniques, the soluble salts can bedissolved in a solvent, for example, nitrates of the rare earth metalsare dissolved in water. Co-precipitation is then obtained by making thesolution basic, e.g., a pH of 9 by adding a base like ammoniumhydroxide. Other soluble metal compounds such as, for example, sulfatesand chlorides, may be used as may mixtures or various soluble compounds,e.g., nitrates with chlorides. The precipitate would then be heated todecompose it to the mixed metal oxide. This heating or calcinationusually would be carried out at temperatures of up to 500° C. It shouldbe noted that the way in which the oxide is obtained for use in formingthe catalyst is not critical to the invention.

In yet another embodiment of this invention, the forward catalyst 70 hasa layered structure, rather than the zoned structure of the previousembodiment. As shown in FIG. 13, the forward catalyst has a top layerwhich contains a catalyst material PM-Rh wherein the precious metal (PM)is selected from the group consisting of platinum, palladium andcombinations thereof. This top layer further includes metal oxidesselected from the group consisting of oxides of aluminum, alkali metals,alkaline earth metals and combinations thereof. The top layer is, in oneexample, Pt and Rh in a 5:1 to 25:1 ratio with a total loading of 60-300g/ft³. This top layer is further optionally stabilized by 2-15% (wt) ofBaO.

The bottom layer includes a catalyst material PM-Rh wherein the preciousmetal (PM) is selected from the group consisting of Pt, Pd andcombinations thereof. This bottom layer can include Pt and Rh in a 1:1to 10:1 ratio, with a total loading of 19-190 g/ft³. The bottom layercan optionally include small amounts of mixed oxides such as zirconiumoxide, cerium oxide and combinations thereof.

As shown in FIG. 13, the bottom layer can contain rhodium anchored on3-5% (wt) ZrO₂, 2-30% (wt) BaO and MgO. In one embodiment, the aluminawashcoat is stabilized by 2-8% (wt) La₂O₃. The alumina washcoat in thesecond bottom layer can optionally be stabilized by composite oxides ofcerium-lanthanum.

In this embodiment, the downstream catalyst 72 has the same formulationsas described above. More specifically, this downstream catalyst 72includes a catalyst material PM-Rh, wherein the precious metal (PM) isPt, Pd or combinations of the two. The downstream catalyst 72 cancontain PM and Rh in a ratio of 5:1 to 15:1 with a total loading of10-700 g/ft³. This catalyst also includes mixed oxides of aluminum,alkali metals, alkaline earth metals or combinations thereof.Additionally, this downstream catalyst 72 includes a high concentrationof mixed oxides of zirconium, cerium or their combination. Both PM andRh can be anchored on 5-30% (wt) high surface area Ce/Zr with high O₂kinetics (e.g., Ce/Zr 50:50 molar ratio). Additionally, in oneembodiment, the alumina washcoat is stabilized by 10% (wt) BaO. Thisdownstream catalyst can also optionally contain suppressants for H₂Semissions such as NiO.

In yet another alternate embodiment of this invention, a single catalystis designed to include multiple zones, where each zone is designed toprovide specific functional advantages. In one embodiment, a catalyst isprovided with two zones, a first-ceria containing zone designed toprovide fast light-off and optimize conversion of HC, CO and NOx understoichiometric conditions, and a second non-ceria containing zoneincluding NOx trapping materials designed to optimize NOx reductionunder lean conditions. This two-zone catalyst provides fast light-off,high activity at stoichiometric conditions and high NOx storagecapabilities at lean conditions. The first ceria-containing zoneprotects the second non-ceria zone from exothermic reactions duringaging and thus improves the durability of the second non-ceria zone. Theceria first zone also provides the ability to desulfate the secondnon-ceria zone. The non-ceria second zone minimizes NOx release duringpurges due to the absence of ceria.

In this two-zone single catalyst embodiment, the first, zone can includealumina, a catalyst mixture PM-Rh where PM is a catalyst materialselected from the group consisting of Pt, Pd and combinations thereof,and metal oxides of zirconium, cerium and combinations thereof. In thisembodiment, the second zone can include alumina, a catalyst mixturePM-Rh as stated above and metal oxides of alkali metals, alkaline earthmetals and combinations thereof.

In another embodiment, a catalyst is designed to include three zones, afirst ceria-containing zone designed to provide fast light-off andoptimize conversion of HC, CO and NOx under stoichiometric conditions,followed by a second non-ceria containing zone which includes NOxtrapping materials designed to optimize NOx reduction under leanconditions, and a third ceria-containing zone with a H₂S suppressant.Once again, the first ceria-containing zone provides fast light-off andhigh activity at stoichiometric conditions. This first ceria-containingzone also protects the second non-ceria zone from exothermic reactionduring aging, increasing durability of the second non-ceria containingzone. The non-ceria containing zone minimizes NOx released duringpurging due to the absence of ceria. The third ceria-containing H₂Ssuppressant zone minimizes the H₂S emissions during desulfations of thesecond zone without re-poisoning the second zone. It should be notedthat the third ceria containing zone could optionally include someadditional NOx storage materials. Additionally, the third zone containsceria and a precious metal PM-Rh to minimize the HC and CO emissionsduring NOx purges and to provide additional activity at stoichiometricconditions.

In this three-zone single catalyst embodiment, the preferable mixturefor the first and second zones is the same as set forth for the two-zonesingle catalyst embodiment. In this three-zone embodiment, the thirdzone can include alumina, a catalyst mixture PM-Rh as stated above,metal oxides of zirconium, cerium and combinations thereof, and hydrogensulfide suppressants.

EXAMPLE 1

Fumed alumina Al₂O₃ (15 g, Degussa), Ba(NO₃)₂ (7.42 g), KNO₃ (0.3 g),La(NO₃)₃.6H₂O (20.3 g), and H₂PtCl₆.6H₂O (1.48 g) are each added to 500ml deionized water, heated to 60° C., and then mixed as solution 1 withthe final desired ratios. A rhodium solution containing 0.10 g Rh(NO₃)₄is deposited onto 2.6 g ZrO₂, dried at 80° C. for eight hours and thencalcined at 600° C. for six hours. The resulting powder is crushed andadded to solution 1. Meanwhile, fumed alumina Al₂O₃ (15 g, Degussa),Ba(NO₃)₂ (14.84 g), and Rh(NO₃)₄ (0.1 g), and H₂PtCl₆.6H₂O (0.23 g) areeach added to 500 ml deionized water, heated to 60° C., and then mixedas solution 2 with the final desired ratios. The top half of a core of400 cell per square inch cordierite (2 inch in diameter and 3.25 inch inlength, zone 1) was dipped into solution 1, and then dried at 80° C.overnight. This process was repeated until the desired amount of mixedcompound was coated on the core. The bottom half of the core (zone 2)was then dipped into solution 2, and then dried at 80° C. overnight.This process was also repeated until the desired amount of compound wascoated on to the bottom half of the core. The coated core was thencalcined in air at 600° C. for six hours.

EXAMPLE 2

This is a comparative example of a known catalyst. Fumed alumina (50.0g, Degussa), and Ba(NO₃)₂ (17.13 g) are dissolved in 500 ml deionizedwater. This mixture is stirred on a hot plate for 1 hour, and then driedat 80° C. overnight, and then calcined at 600° C. for 6 hours. Thecalcined powder is ground with 2.5 g Ce/Zr mixed oxide (W. R. Grace) for48 hours in 190 ml deionized water. Then it is dried at 80° C.overnight, and calcined at 600° C. for six hours. This powder is thenmixed with H₂PtCl₆.6H₂O in 2.65 g deionized water, ground for 12 hours,then dried at 80° C. and calcined at 600° C. for six hours.

EXAMPLE 3

This example shows the test procedures and conditions. The steady statelean NOx trapping efficiency was measured in a flow reactor as anaverage efficiency during a 1 minute lean period under differenttemperatures. The feedgas cycled with 60 seconds lean and 5 seconds richat a constant space velocity of 30,000 hour⁻¹. The flow rates werestrictly controlled by mass flow controllers. The gas concentrationswere measured by a V&F mass spectrometer. The feedgas composition was:

Concentration Concentration Gases (lean) (rich) NO  500 ppm  500 ppm HC1500 ppm 1500 ppm CO 0   4% H₂ 0  1.33%   O₂  6% 0  CO₂ 10% 10% H₂O 10%10% N₂ balance balance

The lambda sweep tests were also conducted in a flow reactor at 400° C.with a gas space velocity of 30,000 hour⁻¹. Here the feedgas compositionremains constant except the oxygen concentration varies to achieve thedesired lambda value. The feedgas contains 2000 ppm HC, 500 ppm NO, 1%CO, 0.33% H₂, 10% CO₂, 10% H₂O, and N₂ balance.

Catalyst aging is conducted in a pulse flame combustor using aproprietary aging cycle for 50 hours with a maximum gas temperature at1000° C.

The foregoing catalyst systems constructions and compositions have beenfound useful in reducing harmful engine emissions. Variations andmodifications of the present invention may be made without departingfrom the spirit and scope of the invention or the following claims. Forexample, the engine may operate at stoichiometry during variousconditions other than just high space velocity or high exhausttemperature, e.g., during starting, during adaptive learning, duringdiagnosis, during default operation if a sensor or component hasdegraded, or various others.

1. A system for an engine, comprising: an upstream catalyst that isadapted to store NOx when the engine is operating lean and release andreduce said stored NOx when operating the engine stoichiometric or rich,said upstream catalyst including a washcoat having a predeterminedamount of precious metal and having NOx storage material, wherein aportion of the predetermined amount of the precious metal is physicallysegregated, in a zone, from the NOx storage material, and where saidupstream catalyst is located in a close coupled location; a downstreamcatalyst that is adapted to store oxidants when the engine is operatinglean and release and reduce said stored oxidants when operating theengine stoichiometric or rich; and a controller for determining whetherexhaust temperature is less than a first value; determining whetherengine airflow is less than a second value; and enabling lean operationbased at least on said first and second determinations.
 2. The systemrecited in claim 1 wherein said exhaust temperature is an exhaust gastemperature estimated from engine operating parameters.
 3. The systemrecited in claim 1 wherein said exhaust temperature is an exhaust gastemperature determined from a temperature sensor in the exhaust system.4. The system recited in claim 1 wherein the upstream catalyst has awashcoat having a predetermined amount of precious metal disassociatedwith NOx storage material.
 5. The system recited in claim 1 wherein saidcontrol enables lean operation based on whether exhaust temperature isless than a first threshold and whether airflow is less than a secondvalue.
 6. The system recited in claim 5 wherein at least one of saidfirst and second threshold are adjusted based on operating conditions.7. A system for an engine, comprising: an upstream emission controldevice and a downstream emission control device in an exhaust system ofthe engine, said upstream device having a washcoat having apredetermined amount of precious metal and having NOx storage material,wherein at least some of the precious metal is physically segregated, ina zone, from the NOx storage material, and where said upstream device islocated in a close coupled location; a controller for adjustingparameters of the engine, said controller operating said engine lean,with periodic rich operation to purge stored NOx, said operation carriedout during at least a portion of low engine airflow and low exhausttemperature conditions; and said controller operating said engine nearstoichiometry during at least a portion of other conditions.
 8. Thesystem recited in claim 7 wherein said upstream emission control deviceis a single brick.
 9. The system recited in claim 7 wherein saidupstream emission control device is multiple bricks in a canister. 10.The system recited in claim 7 wherein said downstream emission controldevice is a single brick.
 11. The system recited in claim 7 wherein saiddownstream emission control device is multiple bricks in a canister. 12.A system for an engine, comprising: an upstream emission control deviceand a downstream emission control device in an exhaust system of theengine, said upstream device with a washcoat having a predeterminedamount of precious metal and having NOx storage material, wherein aselected amount of the precious metal is physically segregated, in azone, from the NOx storage material, said upstream emission controldevice located in a close coupled location receiving untreated exhaustgasses from the engine; a controller for adjusting parameters of theengine, said controller operating said engine lean, with periodic richoperation to purge stored NOx, said operation carried out during atleast a portion of low engine airflow and low exhaust temperatureconditions; and said controller operating said engine near stoichiometryduring at least a portion of other conditions.
 13. The system recited inclaim 12 wherein said amount is between 30 to 70% by mass.
 14. A systemfor an engine, comprising: an upstream catalyst that is adapted to storeNOx when the engine is operating lean and release and reduce said storedNOx when operating the engine stoichiometric or rich, said upstreamcatalyst including a washcoat having precious metal and having NOxstorage material, wherein a selected amount of the precious metalbetween 30% to 70% by mass is physically segregated, in a zone, from theNOx storage material, and where said upstream catalyst is located in aclose coupled location; a downstream catalyst that is adapted to storeoxidants when the engine is operating lean and release and reduce saidstored oxidants when operating the engine stoichiometric or rich; and acontroller for determining whether exhaust temperature is less than afirst value; determining whether engine airflow is less than a secondvalue; and enabling lean operation based at least on said first andsecond determinations.